This invention generally relates to a gas turbine engine, and more particularly to a flutter sensing system for a gas turbine engine.
Gas turbine engines typically include a compressor section, a combustor section and a turbine section. Air is pressurized in the compressor section and is mixed with fuel and burned in the combustor section to add energy to expand the air and accelerate the airflow into the turbine section. The hot combustion gases that exit the combustor section flow downstream through the turbine section, which extracts kinetic energy from the expanding gases and converts the energy into shaft horsepower to drive the compressor section.
In a turbofan gas turbine engine, for example, a fan section is included upstream of the compressor section. Combustion gases are discharged from the gas turbine engine through a core exhaust nozzle and fan air is discharged through an annular fan exhaust nozzle defined at least partially by a nacelle surrounding the core engine. A majority of propulsion thrust is provided by the pressurized fan air which is discharged through the fan exhaust nozzle, while the remaining thrust is provided from combustion gases discharged through the core exhaust nozzle.
A fan section, the compressor section and the turbine section may include multiple airfoils disposed circumferentially about an engine longitudinal centerline axis. At certain aircraft operating conditions, these airfoils may be subjected to flutter, or self-induced oscillations. The flutter conditions are caused by the interaction between adjacent airfoils. During flutter, aerodynamic forces couple with each airfoil's elastic and inertial forces, which may increase the kinetic energy of each airfoil and produce negative damping. The negative damping is enhanced where adjacent airfoils vibrate in unison. Disadvantageously, the airfoil oscillations caused by flutter may become so severe that fracture or failure of the airfoils is possible.
Methods are known for mitigating the negative effects of flutter. For example, many gas turbine engine systems include high pressure compressors having variable vane rows (i.e., vanes that are rotatable about a perpendicular axis relative to a longitudinal centerline axis of the gas turbine engine). The variable vane rows have been used effectively to schedule the engine around flutter conditions by controlling the angle of incidence of the airfoils relative to a direction of flowing airflow. Also, bleed or valve systems are known which bleed airflow downstream from the airfoils to throttle airflow and mitigate flutter. Additionally, airfoil designs are known which tailor a leading edge of each airfoil to obtain improved local airfoil incidence and adjacent airfoils having different natural frequencies. Finally, having inconsistent airfoil spacing in a forward stage varies the intermittent air pulses communicated to a following airfoil stage, thus reducing natural frequency excitation. Disadvantageously, all of these methods result in system compromises, small to moderate performance losses and may be expensive to incorporate into existing gas turbine engine systems.
Accordingly, it is desirable to provide a gas turbine engine having a closed-loop flutter sensing system which achieves reduced flutter operation and minimizes performance losses of the gas turbine engine.